Cell fragments called platelets, or thrombocytes, make up about 2 percent of blood. Platelets stop blood loss from damaged vessels. When a blood vessel tears, platelets at the site adhere to the wall of the vessel to close the tear. The shape of the platelets changes as they liberate the contents of their vesicles. This enables them to connect to one another. Platelets also release chemicals that activate the coagulation system to promote blood clotting. Blood proteins known as clotting factors form fibrin threads. Millions of platelets, together with the fibrin threads, form a platelet plug. If the tear is small enough, the plug can stop blood loss completely.